If an WMI query were to return an empty result set, trying to access the
first element in the result set would result in a `panic: runtime error:
index out of range` error.
add logic to explicitly check if the result set size is 0 and return an
error rather.
Fixes https://github.com/martinlindhe/wmi_exporter/issues/240
the only sql servers I have access to are using the default
`MSSQLSERVER` instance names. I was contacted by someone using a
different instance name and it was failing.
Found 2 bugs in the code:
1) I was returning the default `MSSQLSERVER` as an instance even if it wasn't found in the registry
2) learned that the WMI class naming scheme for non-default instances
was not what I had coded. Changed to incorproate new knowledge.
removed persec from variable names
but when I did that, there was some variable name collisions, so I went
through and name-spaced the class variables.
1
in the process of trying to build a grafana dashboard with "useful"
mssql metrics, I was looking around for what metrics might be useful and
came across an article of [15 SQL Server Performace Counters to Monitor](https://blogs.sentryone.com/allenwhite/sql-server-performance-counters-to-monitor/)
two of the suggested metrics are provided by the AccessMethod
class, which I was not yet capturing.
So I added metrics for the
Win32_PerfRawData_MSSQLSERVER_SQLServerAccessMethods class.
so that the two functions that list available child collectors are next
to each other - should make less likely to not miss editing one of those
when adding additional wmi classes
also removed the 'filter' from the `mssqlFilterAvailableClassCollectors`
function as it's not filtering anyting.
